1. Field of the Invention
This invention relates generally to a regulated DC voltage supply, and more particularly, to a solid state integrated circuit voltage regulator capable of maintaining a substantially constant DC output voltage in the face of temperature variations while exhibiting a low output impedance.
2. Description of the Prior Art
In prior art transistorized voltage regulator circuits, the reference source is typically provided by a zener diode. As is well known, however, zener diodes have inherent characteristics which undesirably restrict the capability of a voltage regulator. An alternate form of solid state regulator which does not utilize a zener diode reference relies, instead, on the temperature dependent characteristics of the base-to-emitter voltage (V.sub.BE) of a transistor. Such an arrangement is shown and described in U.S. Pat. No. 3,617,859. This arrangement, however, suffers from certain serious limitations.
The concept upon which any bandgap reference is based resides in the addition of a .DELTA.V.sub.BE voltage having a positive temperature coefficient to a V.sub.BE having a negative temperature coefficient. The expression for the output voltage in the above cited patent is EQU V.sub.OUT =(kTR.sub.2 /qR.sub.1) ln (I.sub.R /I.sub.O)+V.sub.BE
where k is Boltzman constant, T is the absolute temperature, q is the charge on an electron, R.sub.1 and R.sub.2 are first and second resistors, I.sub.R is a reference current, I.sub.O is the output current and V.sub.BE is a base-emitter voltage of a transistor having a negative temperature coefficient. Since I.sub.O is generally much smaller than the reference current I.sub.R, the expression can be altered to read EQU V.sub.OUT .congruent.(kT/q)(R.sub.2 /R.sub.1)(C)+V.sub.BE
where (C) is substantially constant.
That is, the ratio I.sub.R /I.sub.O will be very large if I.sub.O is much smaller than I.sub.R and therefore ln (I.sub.R /I.sub.O) will not vary considerably with slight variations in current. It can now be seen that by varying the ratio R.sub.2 /R.sub.1, a positive temperature coefficient can be introduced to cancel the negative temperature coefficient associated with V.sub.BE at which point the output voltage V.sub.OUT is substantially equal to the bandgap voltage E.sub.GO. Not only does the expression yield a V.sub.OUT where ln (I.sub.R /I.sub.O) varies slightly with current variations, but the circuit exhibits a poor output impedance.
U.S. Pat. No. 3,887,863 entitled "Solid State Regulated Voltage Supply" overcomes many of the limitations and disadvantages associated with prior art regulators. The circuit disclosed in this patent is a solid state integrated circuit regulated voltage supply which compensates for the effects of changes in temperature and includes first and second transistors having different emitter areas. Equal currents are caused to flow through each of the transistors thus creating different current densities in each of the transistors. Associated circuitry developes a voltage proportional to the .DELTA.V.sub.BE of the two transistors and has a positive temperature coefficient. This voltage is connected in series with the V.sub.BE voltage of one of the two transistors to produce a resultant voltage with a nearly zero temperature coefficient. A negative feedback circuit responsive to current flow through the two transistors automatically adjusts the base voltages to maintain a predetermined ratio of current density for the two transistors. The positive temperature coefficient current component in this case is EQU (kT/qR) ln [N(I.sub.1 /I.sub.2)]
where N represents the emitter area ratio and I.sub.1 and I.sub.2 are the respective currents flowing through the first and second transistors. Now, since I.sub.1 and I.sub.2 are equal, the relationship becomes EQU (kT/qR) ln N.
The natural logrithm term is now clearly a constant. The unity gain feedback output impedance (R.sub.OUT) is generally equal to EQU R.sub.OUT =1/gm
where gm is the open loop transconductance of the amplifier. The gm of the circuit shown in the above cited patent is generally much lower than a differential type amplifier due to emitter degeneration resistance and thus exhibits poor output impedance in comparison with the differential type. Furthermore, the closed loop output resistance and the high frequency bandwidth deteriorates when resistive dividers are employed to level shift the reference voltage to higher output voltages due to reduction in negative feedback caused by the resistive level shift.